Recently, a nonaqueous electrolyte battery such a lithium ion secondary battery has been frequently researched and developed as a battery having a high energy density. The nonaqueous electrolyte battery is expected to be used as a power source for vehicles such as hybrid vehicles, electric vehicles, or an uninterruptible power supply for mobile phone base stations. The capacity of the lithium ion secondary battery is likely to increase further. Accordingly, it is important to ensure the safety of the lithium ion secondary battery.
In ordinary lithium ion secondary batteries, a polyolefin-based porous film is used as a separator that is interposed between a positive electrode and a negative electrode. For example, polyethylene having a relatively low melting point is used as a polyolefin-based material. This is because it ensures a so-called shutdown effect where the constituent resin of the separator is allowed to close the pores by melting at a temperature equal to or lower than the thermal runaway temperature of a battery, thereby increasing the internal resistance of the battery to improve the safety of the battery at the time of a short circuit.
The above separator is produced by uniaxially or biaxially orienting a film in order to improve, for example, the porosity and strength. Since a separator is provided as stated above, a certain level of strength is ensured in view of workability as an independent film. In such a uniaxially- or biaxially-oriented film, however, the degree of crystallinity is increased, and the level of the shutdown temperature is also increased close to the thermal runaway temperature of the battery. For that reason, it is hard to say that the safety of the battery is sufficient.
Further, the film subjected to drawing may shrink due to residual stress when being subjected to high temperatures.
In order to solve the problems, there is, for example, a technique for forming an integral separator layer having an excellent heat resistance on the surface of an electrode. The integral separator has various forms. The use of metal oxide particles achieves a separator having an excellent heat resistance. The integral separator does not need the same strength as that of an ordinary separator. For that reason, the thickness of the integral separator is decreased, thereby increasing the volume energy density of the battery.
Further, the separator formed of the metal oxide particles is not shrunk by heating. Thus, the separator can prevent expansion of a short circuit when an internal short circuit is generated, and prevent abnormal heat when the battery is subjected to high temperatures, thereby providing excellent safety.
However, a contact area between the particles and active material particles constituting the integral separator is large, whereby the internal resistance becomes large and a discharge performance at low temperatures is reduced.